In the LTE (Long Term Evolution), a state in which communications via a radio link can be performed between a radio base station eNB and a mobile station UE is referred to as an “RRC_CONNECTED state”.
Note that a state which is not the “RRC_CONNECTED state” is referred to as an “RRC_IDLE state”. In the “RRC_IDLE state”, the radio base station eNB is not aware of the presence of the mobile station UE, and therefore communications via a radio link cannot be performed between the radio base station eNB and the mobile station UE.
In the LTE, in order to reduce power consumption of the mobile station UE, the mobile station UE in the “RRC_CONNECTED state” is capable of performing discontinuous reception (DRX) of signals transmitted from the radio base station eNB.
FIG. 1 (a) to FIG. 1 (c) show DRX control performed in the “RRC_CONNECTED state”, on which an agreement has been made in the LTE.
The discontinuous reception is characterized by a “DRX cycle” and an “On duration length (discontinuous reception timing duration)”.
The mobile station UE is configured to receive a signal, transmitted from the radio base station eNB, only during the “On duration (discontinuous reception timing)” occurring once in a “DRX cycle”.
Since no signal is transmitted from the radio base station eNB in a time period other than the “On duration”, the mobile station UE turns off a processor, a DSP, a chip, and the like which are used for radio signal reception processing, thereby achieving reduction in power consumption.
Note that the radio base station eNB is configured to notify the mobile station UE of the “DRX cycle” and the “On duration length”.
Conceivable examples of a timing at which the radio base station eNB notifies the mobile station UE of the “DRX cycle” and the “On duration length” include a timing at which the “RRC_IDLE state” transitions to the “RRC_CONNECTED state”, a timing at which a new radio bearer is set up, a timing at which a radio bearer is released, a handover timing, and the like.
In addition, when detecting radio resource allocation through a radio resource allocation channel (L1/L2 control channel) from the radio base station eNB, in the “On duration” during the discontinuous reception as shown in FIG. 1 (b), the mobile station UE is configured to start continuous reception of signals transmitted from the radio base station eNB.
Specifically, when detecting a mobile station identifier allocated to the mobile station UE as a result of decoding of the L1/L2 control channel, the mobile station UE performing the discontinuous reception is configured to start the continuous reception of signals transmitted from the radio base station eNB (T1 in FIG. 1 (b)).
On the other hand, when not detecting the radio resource allocation through the L1/L2 control channel from the radio base station eNB within a certain continuous period (that is, a period until an inactivity timer (inactive timer) 1 expires) during the continuous reception as shown in FIG. 1 (c), the mobile station UE is configured to start discontinuous reception of signals transmitted from the radio base station eNB (T2 in FIG. 1 (c)).
In the LTE, at least two types of DRX cycles can be set as a DRX cycle in discontinuous reception.
In this respect, the mobile station UE is configured to change the DRX cycle from a DRX cycle on a first phase (DRX cycle (short)) to a longer DRX cycle on a second phase (DRX cycle (long) when not detecting the radio resource allocation through the L1/L2 control channel from the radio base station eNB within an additional certain continuous period (that is, a period until an inactivity timer 2 expires) (T3 in FIG. 1 (c)).
Note that, the “DRX cycle (short)” and the “DRX cycle (long)” are notified from the radio base station eNB to the mobile station UE.
Here, if one of the DRX cycles (“DRX cycle (short)” or “DRX cycle (long)”) is set invalid, for example, the DRX cycle in the discontinuous reception can be set to have only one phase.
In addition, periods managed by the inactivity timers 1 and 2 are also notified from the radio base station eNB to the mobile station UE.
Conceivable examples of a timing at which the radio base station eNB notifies the mobile station UE of the periods managed by the inactivity timers 1 and 2 include a timing at which the “RRC_IDLE state” transitions to the “RRC_CONNECTED state”, a timing at which a new radio bearer is set up, a timing at which a radio bearer is released, a handover timing, and the like.
Parameters related to the DRX control in the “RRC_CONNECTED state” in the LTE are listed below.
1. DRX cycle (short) and DRX cycle (long)
2. On duration length
3. Periods managed by inactivity timers 1 and 2
Optimum values of these parameters vary depending on the type of an application for processing data (for example, voice packets, burst traffic data, or the like) transmitted and received between the radio base station eNB and the mobile station UE.
For example, as shown in FIG. 2, when communications are performed by an application for voice packets, voice packets having small-volume data occur at predetermined intervals.
Here, when communications are performed by the application for voice packets, the voice packets occur every 20 ms. Generally, the voice packets each have data volume which can be transmitted and received within one sub-frame (also referred to as TTI).
In addition, the sub-frame is the minimum time unit at which a radio resource can be allocated, and is 1 ms in the LTE.
Furthermore, in view of the QoS properties required by the application for voice packets, a voice packet transmission delay needs to be suppressed as much as possible. It is therefore general that the radio base station eNB transmits a voice packet to the mobile station UE immediately after an occurrence thereof.
For this reason, in the case of the application for voice packets, it seems optimum to set the parameters related to the DRX control as follows for example.
1. DRX cycle: 20 ms (only “short” is valid)
2. On duration length: one sub-frame
3. Period managed by an inactivity timer 1: one sub-frame
Meanwhile, as shown in FIG. 3, there also exist applications for burst traffic data in which traffic data (packets) occur in a burst, such as applications for viewing the Internet, and for transmitting and receiving a file through an FTP.
The applications for burst traffic data have such features that even though a large volume of traffic data occurs when the traffic data occurs, there is a long time period during which no traffic data occurs at all.
In addition, it is often the case that these applications for burst traffic data do not have a very strict requirement for a packet transmission delay. Accordingly, even when burst traffic data occurs, the radio base station eNB does not have to transmit the burst traffic data to the mobile station UE immediately. The radio base station eNB is allowed to transmit the burst traffic data by selecting, for example, time when a radio link between the radio base station eNB and the mobile station UE has a good quality.
Accordingly, for these applications for burst traffic data, when burst traffic data exists in the radio base station eNB, it is desirable that the mobile station UE perform continuous reception as continuously as possible. It thus seems optimum to set the parameters related to the DRX control as follows, for example.
1. DRX cycle: several hundred ms (short), and a value in the order of seconds (long)
2. On duration length: several sub-frames
3. Periods managed by inactivity timers: several hundred ms (1), and several seconds to several ten seconds (2)
Non-Patent Document 1: 3GPP TS36.300 V8.1.0, June 2007
As learned from the above-described example, the optimum values of the parameters related to the DRX control vary considerably depending on the type of an application for processing data transmitted and received between the radio base station eNB and the mobile station UE.
Accordingly, the DRX control on which an agreement is made in the LTE has a problem that optimum DRX control cannot be performed in a case where, for example, both types of data of an application for voice packets and an application for burst traffic data are concurrently transmitted or received between the radio base station eNB and the mobile station UE.
In such a case, if the parameters related to the DRX control are set to values optimized, for example, for the application for voice packets, data for the other application in which burst traffic data occurs cannot be handled optimally.
Specifically, as shown in FIG. 4, since the period managed by the inactivity timer and optimized for the application for voice packets is too short, the mobile station UE starts discontinuous reception if the radio base station eNB stops, even for an instant, transmitting data to the mobile station UE performing continuous reception. This eliminates the flexibility in timing at which the burst traffic data can be transmitted to the mobile station UE, thus preventing optimum radio resource utilization.
On the other hand, if the period managed by the inactivity timer is set longer, the mobile station UE transitions to a continuous reception state every time an L1/L2 control channel is used for voice packet transmission, and then returns to a discontinuous reception state (DRX state) at a much later timing. Thus, power consumption of the mobile station UE cannot be fully reduced.
Meanwhile, it has been determined that the LTE supports a radio resource allocation method referred to as “persistent scheduling (static allocation scheduling)” or “semi-persistent scheduling)” so as to transmit voice packets efficiently.
By contrast, a radio resource allocation method normally employed in the LTE is referred to as “dynamic scheduling (dynamic allocation scheduling)”.
When allocating a downlink radio resource to the mobile station UE by using the dynamic scheduling, the radio base station eNB transmits a radio resource allocation channel (L1/L2 control channel) to the mobile station UE. The L1/L2 control channel is terminated at the physical layer and MAC layer of each of the radio base station eNB and the mobile station UE. Meanwhile, the mobile station UE receives the L1/L2 control channel, thereby recognizes that the radio resource is allocated to the mobile station UE, and decodes data transmitted by using the radio resource allocated to the mobile station UE, in accordance with information included in the received L1/L2 control channel.
Specifically, the mobile station UE is configured to attempt to perform decoding processing on the L1/L2 control channel during the “On duration” when performing discontinuous reception, and configured to attempt to perform decoding processing on the L1/L2 control channel every sub-frame when performing continuous reception.
The L1/L2 control channel for notifying downlink radio resource allocation using the dynamic scheduling includes information such as data block size (TB size), a modulation method (modulation), an allocated physical resource (PRB) and HARQ-related information. Based on these kinds of information, a CRC bit sequence is calculated.
For the mobile station UE targeted for radio resource allocation, the calculated CRC bit sequence is further subjected to a predetermined operation (masking) using a mobile station identifier previously and uniquely allocated to the mobile station UE, and then added to an information bit sequence of the L1/L2 control channel.
Subsequently, the information bit sequence of the L1/L2 control channel including the CRC bit sequence is subjected to error correction coding processing, and then transmitted as a radio signal to the mobile station UE.
The mobile station UE is configured to receive a physical resource through which the L1/L2 control channel is transmitted, to perform error correction decoding processing thereon, and then to perform a predetermined operation (unmasking) on the CRC bit sequence by using the mobile station identifier allocated by the radio base station eNB beforehand.
Then, based on a CRC determination result based on the CRC bit sequence resulting from the predetermined operation, the mobile station UE is configured to determine whether or not the L1/L2 control channel has been properly decoded.
Here, in a case where the L1/L2 control channel transmitted to a different mobile station UE from the radio base station eNB is transmitted to the mobile station UE, there is a mismatch between mobile station identifiers, one of which is used in the predetermined operation (unmasking) on the CRC bit sequence by the mobile station UE after the error correction decoding, and the other of which is used in the predetermined operation (masking) on the CRC bit sequence by the radio base station eNB before the error correction decoding. Accordingly, the mobile station UE can determine that the L1/L2 control channel has not been decoded properly on the basis of the CRC determination result based on the unmasked CRC bit sequence.
This means that, when determining that the L1/L2 control channel notifying the mobile station UE of the downlink radio resource allocation has been decoded properly on the basis of the CRC determination result, the mobile station UE concurrently detects that the downlink radio resource is allocated to the mobile station UE.
A downlink radio resource allocated by using the dynamic scheduling is valid only within one sub-frame. In order to allocate a downlink radio resource to a specific mobile station UE over different sub-frames, an L1/L2 control channel for downlink radio resource allocation needs to be transmitted to the mobile station UE every sub-frame, as shown in FIG. 5.
In sum, since a downlink radio resource to be allocated can be changed sub-frame by sub-frame by using the dynamic scheduling, optimum radio resource allocation can be performed according to the ever-changing quality and data volume of a radio link.
Meanwhile, there is a problem of increasing overhead of a downlink radio resource, if the dynamic scheduling is employed for an application, such as an application for voice packets, in which packets each having certain small-volume data occur regularly. This is because the L1/L2 control channel itself consumes a downlink radio resource.
In this respect, in order to support such an application for voice packets efficiently, the above-described radio resource allocation method using the persistent scheduling has been studied and determined to be supported in the LTE.
When allocating a downlink radio resource to the mobile station UE by using the persistent scheduling, the radio base station eNB notifies, to the mobile station UE, information such as data block size (TB size), a modulation method (modulation), an allocated physical resource (PRB) and HARQ-related information. For the notification, the radio base station uses either a radio resource allocation channel (L1/L2 control channel) terminated at the physical layer and MAC layer of each of the radio base station eNB and the mobile station UE, or an RRC message terminated at the RRC layer of each of the radio base station eNB and the mobile station UE.
Here, a value of each of these kinds of notified information is not limited to a single value, but plural candidate values may be notified as appropriate.
The mobile station UE is configured to perform decoding of the downlink radio resource in a certain sub-frame in accordance with these kinds of information even without receiving the radio resource allocation through the L1/L2 control channel.
As shown in FIG. 6, the radio resource allocation by the persistent scheduling is valid until a new persistent scheduling command or a command for stopping the persistent scheduling occurs.
Here, when the radio base station eNB uses an L1/L2 control channel to notify, to the mobile station UE, a downlink radio resource allocated by the persistent scheduling, the mobile station UE needs to discriminate between the L1/L2 control channel used for notifying the downlink radio resource allocated by the persistent scheduling and an L1/L2 control channel used for notifying a downlink radio resource allocated by the dynamic scheduling.
In addition, it has been determined, in the LTE, that, even though downlink radio resource allocation by the persistent scheduling is employed, a downlink radio resource to be used for retransmission under HARQ control should be notified by using an L1/L2 control channel every time retransmission under HARQ control is performed.
In this case, the mobile station UE also needs to discriminate between the L1/L2 control channel for notifying the downlink radio resource to be used for the retransmission under HARQ control in the persistent scheduling and an L1/L2 control channel used for notifying a downlink radio resource allocated by the dynamic scheduling.
As described above, the existing LTE has a problem that, when both types of data of an application for voice packets and an application for burst traffic data are transmitted or received between the radio base station eNB and the mobile station UE, it is difficult to optimize the parameters related to the DRX control in the “RRC_CONNECTED state”.